Silicas

ABSTRACT

Silanised, structurally modified silicas, characterised by groups fixed on the surface, the groups being alkylsilyl (SiC n H 2n+1 , with n=2-18). They are produced in that pyrogenically produced silicas are treated with the silanising agent and structurally modified. They are used to improve scratch resistance in lacquers.

The invention relates to silanised, structurally modified, pyrogenically produced silicas, a process for the production thereof and their use.

Silanised silicas are used as thickeners, such as e.g. for water-thinnable lacquers and resins, such as e.g. epoxy resins.

From EP 0 672 731 B1, silanised, pyrogenically produced silicas are known, which are characterised in that the pyrogenically produced silicas are treated with a compound from the group (RO)₃SiC_(n)H_(2n+1), wherein n=10 to 18 and R=short-chained alkyl radicals. For example, the pyrogenically produced silicas have been treated with the compound (CH₃O)₃SiC₁₆H₃₃ (hexadecyltrimethoxysilane) or with the compound (CH₃O)₃SiC₁₈H₃₇ (octadecyltrimethoxysilane).

The production of the silanised, pyrogenically produced silicas takes place in that the pyrogenically produced silicas are placed in a mixer, the silicas are sprayed, optionally first with water and then with the compound from the group (RO)₃SiC_(n)H_(2n+1) while mixing intensively, mixed for a further 15 to 30 minutes and then tempered at a temperature of 100 to 160° C. for a period of 1 to 3 hours.

The invention provides silanised, structurally modified, pyrogenically produced silicas characterised by groups fixed on the surface, the groups being alkylsilyl (SiC_(n)H_(2n+1), with n=2-18), preferably octylsilyl and/or hexadecylsilyl.

The silica according to the invention can have the following physico-chemical characteristics:

BET-surface area m2/g: 25-400

-   Average size of the primary particles nm: 5-50 -   pH value: 3-10 -   Carbon content %: 0.1-25 -   DBP value %: The DBP value is at least 10% lower than the DBP value     of the corresponding silanised, non-structurally modified silica.     With very marked structural modification, the structure can be     broken down in such a way that the DBP value can no longer be     determined.

A silica produced by a high-temperature hydrolysis route from SiCl₄+H₂ and O₂ can be used as the pyrogenically produced silica.

In particular, a silica produced by high temperature hydrolysis having the following physico-chemical characteristics can be used: TABLE 1 AEROSIL AEROSIL AEROSIL AEROSIL AEROSIL AEROSIL AEROSIL AEROSIL 90 130 150 200 300 380 OX 50 TT 600 Behaviour in respect of water hydrophilic Appearance loose white powder BET surface area¹) m²/g 90 ± 15 130 ± 25 150 ± 15 200 ± 25 300 ± 30 380 ± 30 50 ± 15 200 ± 50 Average size of the nm 20 16 14 12 7 7 40 40 primary particles Tamped density²) standard material g/l ca. 80 ca. 50 ca. 50 ca. 50 ca. 50 ca. 50 ca. 130 ca. 60 compacted material g/l — ca. 120 ca. 120 ca. 120 ca. 120 ca. 120 — — (additive “V”) Loss on drying³) <1.0 <1.5 <0.5⁹) <1.5 <1.5 <1.5 <1.5 <2.5 (2 hours at 1000° C.) % on leaving supplier's works Loss on ignition⁴)⁷) % <1 <1 <1 <2 <2 <2.5 <1 <2.5 (2 hours at 1000° C.) pH value⁵) (in 4% 3.6-4.5 3.6-4.3 3.6-4.3 3.6-4.3 3.6-4.3 3.6-4.3 3.8-4.8 3.6-4.5 aqueous dispersion) SiO₂ ⁸) % >99.8 >99.8 >99.8 >99.8 >99.8 >99.8 >99.8 >99.8 Al₂O₃ ⁸) % <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.08 <0.05 Fe₂O₃ ⁸) % <0.003 <0.003 <0.003 <0.003 <0.003 <0.003 <0.01 <0.003 TiO₂ ⁸) % <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 HCl⁸)⁹) % <0.025 <0.025 <0.025 <0.025 <0.025 <0.025 <0.025 <0.025 Sieving residue⁶) % <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.2 <0.05 (acc. to Mocker, 45 μm) ¹)based on DIN 66131 ²)based on DIN ISO 787/XI, JIS K 5101/18 (not sieved) ³)based on DIN ISO 787/II, ASTM D 280, JIS K 5101/21 ⁴)based on DIN 55 921, ASTM D 1208, JIS K 5101/23 ⁵)based on DIN ISO 787/IX, ASTM D 1208, JIS K 5101/24 ⁶)based on DIN ISO 787/XVIII, JIS K 5101/20 ⁷)based on the substance dried for 2 hours at 105° C. ⁸)based on the substance ignited for 2 hours at 1000° C. ⁹)HCl content is a component of the loss on ignition

Pyrogenic silicas of this type are known. They are described, inter alia, in:

-   Winnacker-Küchler, Chemische Technologie, volume 3 (1983), 4^(th)     edition, page 77 and -   Ullmanns Encyklopädie der technischen Chemie, 4^(th) edition (1982),     volume 21, page 462.

The pyrogenically produced silicas are treated with a compound from the group (RO)₃SiC_(n)H_(2n+1), wherein n=2 to 18 and R=alkyl, such as e.g. methyl, ethyl or similar.

In particular, the following compounds can be used:

-   Silane I (CH₃O)₃SiC₁₆H₃₃ (hexadecyltrimethoxysilane) -   Silane II (CH₃O)₃SiC₈H₁₇ (octyltrimethoxysilane)

The silicas according to the invention can be produced in that the pyrogenically produced silicas are placed in a mixer, the silicas are sprayed, optionally first with water and then with the compound (organosilane) from the group (RO)₃SiC_(n)H_(2n+1) while mixing intensively, mixed for a further 15 to 30 minutes and then tempered at a temperature of 100 to 160° C. for a period of 1 to 3 hours, structurally modified and/or optionally post-ground. A further tempering can optionally take place after the structural modification and/or post-grinding.

The structural modification can take place e.g. with a ball mill or a continuously operating ball mill. The post-grinding can take place e.g. using an air-jet mill or pin mill. The tempering can take place batchwise, e.g. in a drying cupboard, or continuously, e.g. in a fluidised bed. The tempering can take place under protective gas, e.g. nitrogen.

The water used can be acidified with an acid, e.g. hydrochloric acid, down to a pH value of 7 to 1.

The organosilane used can be dissolved in a solvent, such as e.g. ethanol.

The tempering can be performed in a protective gas atmosphere, such as e.g. under nitrogen.

The pyrogenically produced silicas according to the invention silanised with silane I have the physico-chemical characteristics listed in Table 2 before structural modification: TABLE 2 Educt A 90 A 130 A 150 A 200 A 300 A 380 OX 50 TT 600 Average size of the 20 16 14 12  7  7 40 40 primary particles [nm] BET surface area 40-90  60-130  75-150 100-200 150-300 200-380 20-50 100-250 [m²/g] Tamped density  40-140  40-140  40-140  40-140  40-140  40-140  40-140  40-140 [g/l] Loss on drying [%] <2 <2 <2 <2 <2 <2 <2 <2 Loss on ignition 0.1-10  0.1-10  0.1-10  0.5-15  0.5-20  0.5-25  0.1-10  0.1-20  [%] C content [%] 0.1-10  0.1-10  0.1-10  0.5-15  0.5-20  0.1-25  0.1-10  0.5-20  pH value 3.5-5.5 3.5-5.5 3.5-5.5 3.5-5.5 3.5-5.5 3.5-5.5 3.5-5.5 3.5-5.5

The silanised, structurally modified, pyrogenically produced silicas according to the invention can be used to improve scratch resistance in lacquers.

EXAMPLES

The pyrogenically produced silicas used have the physico-chemical characteristics listed in Table 1.

As organosilanes, the following compound with the general formula (RO)₃SiC_(n)H_(2n+1) is used:

-   (Silane I) (CH₃O)₃SiC₁₆H₃₃

The silica is placed in a mixer and sprayed first with water and then with organosilane, mixing intensively.

When the spraying is complete, stirring is continued for a further 15 to 30 minutes and then the mixture is tempered for 1 to 3 hours at 100 to 160° C. The tempering can also take place under protective gas, e.g. nitrogen.

The individual reaction conditions can be taken from Table 3.

The physico-chemical characteristics of the silanised silicas obtained are listed in Table 4. TABLE 3 Tem- Silane Water Ethanol Tem- pering Ex- quantity quantity quantity pering temper- am- (g/100 g (g/100 g (g/100 g period ature ple Aerosil Silane Aerosil) Aerosil) Aerosil) (h) (° C.) 1 A 300 Silane I 1 0 9 2 120 2 A 200 Silane I 2.5 0 0 2 140 3 A 200 Silane I 20 5 0 2 140 4 A 200 Silane I 10 2.5 0 2 140 5 A 200 Silane I 5 1.25 0 2 140 6 A 200 Silane I 2.5 1.25 0 2 140

TABLE 4 Tamped Surface Loss on Loss on density C content area drying ignition Example pH value (g/l) (%) (m²/g) (%) (%) 1 4.3 50 1.3 253 0.4 1.8 2 4.4 49 1.7 176 0.3 2.5 3 4.6 68 10.1 116 0.6 12.7 4 4.5 72 5.7 144 0.6 7.1 5 4.7 52 2.6 167 0.6 3.4 6 4.5 51 1.9 171 0.7 2.5 Production and Physico-Chemical Properties of the Silicas According to the Invention Production of the Silicas According to the Invention:

The silicas, which can be produced as described in EP 0 672 731, are then structurally modified by mechanical action and possibly post-ground in a mill. A tempering can possibly take place after the structural modification and/or post-grinding.

The structural modification can take place e.g. with a ball mill or a continuously operating ball mill. The post-grinding can take place e.g. using an air-jet mill or pin mill. The tempering can take place batchwise, e.g. in a drying cupboard, or continuously, e.g. in a fluidised bed. The tempering can take place under protective gas, e.g. nitrogen. TABLE 5 Overview of the production of the comparative silicas and the silicas according to the invention (Examples) Post- grinding after Tempering Surface-fixed Structural structural after Designation group modification modification post-grinding Comparative Hexadecylsilyl No — — silica 1 Comparative Octylsilyl No — — silica 2 Silicas 1 Hexadecylsilyl Yes No No Silicas 2 Octylsilyl Yes Yes No Silicas 3 Hexadecylsilyl Yes Yes Yes Silicas 4 Octylsilyl Yes No Yes Silicas 5 Octylsilyl Yes Yes No Silicas 6 Hexadecylsilyl Yes Yes No Silicas 7 Hexadecylsilyl Yes Yes No Silicas 8 Hexadecylsilyl Yes No No Silicas 9 Octylsilyl Yes Yes No Silicas 10 Octylsilyl Yes No No Silicas 11 Octylsilyl Yes Yes No Silicas 12 Octylsilyl Yes No No

TABLE 6 Physico-chemical data of the silicas according to the invention (Examples) and the comparative silicas Tamped Loss on DBP BET specific density Loss on ignition adsorption surface area Designation [g/l] drying [%] [%] pH value C content [%] [%] [m²/g] Comparative silica 1 57 0.5 1.8 4.6 1.2 302 195 Comparative silica 2 51 0.6 6.8 5.3 5.4 263 175 Silicas 1 137 0.7 1.9 4.9 1.3 217 193 Silicas 2 112 0.7 7.0 5.8 5.5 145 175 Silicas 3 118 0.7 2.3 5.1 1.3 228 176 Silicas 4 163 0.9 6.7 5.3 5.4 134 176 Silicas 5 114 0.5 7.1 6.0 5.4 142 175 Silicas 6 113 1.3 2.2 5.1 1.4 221 193 Silicas 7 123 0.7 2.6 6.0 1.4 208 197 Silicas 9 146 1.1 2.3 5.8 1.4 182 195 Silicas 9 240 0.8 6.7 4.8 5.3  87 169 Silicas 10 322 0.3 6.9 6.0 5.3 Could not 172 be determined Silicas 11 204 0.7 6.4 5.7 5.4 101 173 Silicas 12 276 0.3 6.6 6.6 5.3 Could not 168 be determined

APPLICATION EXAMPLES Example 1

For the investigation of the improvement in scratch resistance, a conventional 2-component polyurethane lacquer was used. The formulation of the lacquer and its production, including application, are summarised below:

Formulation: Parts by wt. Millbase Acrylic resin, 50% in xylene/ethylbenzene 3:1 53.3 Butyl acetate 98% 6.7 Xylene 6.7 Silica 5.0 Σ 71.7 Lacquer make-up Acrylic resin, 50% in xylene/ethylbenzene 3:1 1.1 Xylene 12.2 Ethoxypropyl acetate 1.5 Butyl glycol acetate 1.5 Butyl acetate 98% — Aliphatic polyisocyanate, approx. 75% in 1- 17.0 methoxypropyl-2-acetate/xylene 1:1 Σ 105.0

-   Binder concentration: 40% -   Silica calculated on the basis of millbase (solids): 18.8% -   Silica calculated on the basis of lacquer (total): 5.0% -   Silica calculated on the basis of lacquer (solids): 12.5%     Production and Application of Lacquers

The binder is mixed with the solvents. Then, for the purpose of predispersion, the silica is incorporated into this mixture with the high-speed mixer (disk Ø 45 mm) and predispersed for 5 min at 2000 rpm. The mixture is dispersed in a laboratory pearl mill for 30 min at 2500 rpm and 60% pump capacity using glass beads (Ø approx. 1 mm). The millbase is tested with a grindometer, 25 μm, according to DIN ISO 1524. It must be smaller than 10 μm.

The conversion of the millbase to lacquer takes place in accordance with the formulation, the components being mixed at 2000 rpm with a vane agitator. The hardener is incorporated in the same way.

After adjusting the lacquers to spray viscosity in accordance with DIN 53411, the lacquers are applied to black lacquered metal sheets, e.g. DT 36 (from Q-Panel), by spray application (coat thickness about 40-50 μm). After spraying, the metal sheets are dried for 24 h at room temperature and then for 2 h in a drying oven at 70° C.

Scratch Tests:

The metal sheets are abraded with a quartz/water slurry (100 g water+1 g Marlon A 350, 0.25%+5 g Sikron F500) and with a CaCO₃/water mixture (100 g water+1 g Marlon A 350, 0.25%+5 g Millicarb BG) using an abrasion and washing resistance tester (Erichsen, brush with hog's bristles). The gloss before and 10 min after the abrading is determined with a reflectometer (200 irradiation angle). TABLE 7 Summary of the properties of the liquid lacquers relevant in terms of lacquer technology, and of the applied and dried films. Reference Reference Comparative without Comparative Silica without silica 1 Silica 7 Silica 8 silica silica 2 Silica 9 11 silica Grindometer value [μm] <10 <10 <10 <10 <10 <10 <10 / Viscosity (millbase) [mPas]  6 Rpm 409 210 220 / 5670 935 832 / 60 Rpm 407 210 212 / 1260 409 407 / Viscosity (lacquer + hardener) [mPas]  6 rpm 120 80 80 60 446 195 175 55 60 rpm 113 82 82 61 194 146 144 64 Flow poor OK OK OK Orange peel OK OK OK fine cracks effect Scratch resistance 20° reflectometer value 81 89.5 89.1 91.3 38 85.5 85.3 91.7 before scratching Haze before scratching 101 9 12 2 423 18 19 2 Black value My 272 286 286 291 260 283 282 294 40 strokes with Sikron 83.4 88.5 90.7 51.8 / 80.4 84.3 56.1 F 500 residual gloss [%]

The silicas 7+8 and 9+11 according to the invention can be used in high concentrations without impairing the appearance of the lacquer surface owing to their substantially lower rheological efficiency compared with comparative silica 1+2. In addition, the silicas according to the invention display a substantial improvement in scratch resistance of the lacquer surface.

Example 2

In this example the influence of the structural modification was investigated on the basis of a high solids 2-component PU clear lacquer. The formulation of the lacquer and its production, including application and testing, are summarised below:

Formulation: Parts by wt. Millbase Acrylic copolymer, mod. with synthetic 61.0 fatty acids, 70% in n-butyl acetate Butyl acetate 98% 7.3 Methoxypropyl acetate 1.7 Solvesso 100 2.0 Xylene 2.0 Baysilon OL 17, 10% in xylene (silicone 0.7 oil) Silica 5.0 Σ 79.7 Lacquer make-up (hardener) Aliphatic polyisocyanate, 90% in n- 22.3 butyl acetate Butyl acetate 98% 2.0 Solvesso 100 1.0 Σ 105.0

-   Binder concentration: 62.8% -   Silica calculated on the basis of millbase (solids): 11.7% -   Silica calculated on the basis of lacquer (total): 5.0% -   Silica calculated on the basis of lacquer (solids): 8.0%     Production and Application of the Lacquers

The binder is mixed with the solvents. Then, for the purpose of predispersion, the silica is incorporated into this mixture with the high-speed mixer (disk Ø 45 mm) and predispersed for 5 min at 2000 rpm. The mixture is dispersed in a laboratory pearl mill for 30 min at 2500 rpm and 60% pump capacity using glass beads (Ø approx. 1 mm). The millbase is tested with a grindometer, 25 μm, in accordance with DIN ISO 1524. It must be smaller than 10 μm.

The conversion of the millbase to lacquer takes place in accordance with the formulation, the components being mixed with a vane agitator at 2000 rpm. The hardener is incorporated in the same way.

After adjusting the lacquers to spray viscosity in accordance with DIN 53411, the lacquers are applied to black lacquered metal sheets, e.g. DT 36 (from Q-Panel), by spray application (coat thickness about 40-50 μm). After spraying, the metal sheets are dried for 24 h at room temperature and then for 2 h in a drying oven at 70° C.

Scratch Tests:

The metal sheets are abraded with a quartz/water slurry (100 g water+1 g Marlon A 350, 0.25%+5 g Sikron F500) using an abrasion and washing resistance tester (Erichsen, brush with hog's bristles). The gloss before and 10 min after the abrading is determined with a reflectometer (20° irradiation angle). TABLE 8 Summary of the properties of the liquid lacquers relevant in terms of lacquer technology, and of the applied and dried films. Reference Comparative Silica without silica 1 silica 7 8 Silica Bulk density [g/l] 50 146 123 / Grindometer value [μm] <10 <10 <10 / Viscosity (millbase) [mPas]  6 rpm 767 376 376 205 60 rpm 717 359 361 205 Viscosity (lacquer + hardener) [mPas]  6 rpm 459 279 281 120 60 rpm 399 272 274 120 Flow poor (fine OK OK OK “cracks”) Scratch resistance 20° reflectometer 82.3 86.5 86.3 88.2 value before scratching Haze before scratching 3 4 4 2 Black value My 275 283 282 292 40 strokes with 63.2 78.2 75.4 30.2 Sikron F 500 residual gloss [%]

The silicas 7+8 according to the invention can be used in high concentrations without impairing the appearance of the lacquer surface owing to their substantially lower Theological efficiency compared with comparative silica 1. In addition, the silicas according to the invention display a substantial improvement in the scratch resistance of the lacquer surface.

Example 3

For the investigation of the improvement of the scratch resistance, a conventional 2-component polyurethane lacquer was used. The formulation of the lacquer and its production, including its application, are summarised below:

Formulation Parts by wt. Millbase Acrylic copolymer, mod. with 43.4 synthetic fatty acids, 60% solution Butyl acetate 98% 17.8 Xylene 3.9 Silica 5.0 Σ 70.7 Lacquer make-up Xylene 11.3 Ethoxypropyl acetate 3.4 Butyl glycol acetate 1.6 Aliphatic polyisocyanate, approx. 18.6 75% in 1-methoxypropyl-2- acetate/xylene 1:1 Σ 105.0

-   Binder concentration: 40% -   Silica calculated on the basis of millbase (solids): 19.2% -   Silica calculated on the basis of lacquer (total): 5.0% -   Silica calculated on the basis of lacquer (solids): 12.5%     Production and Application of the Lacquers

The binder is mixed with the solvents. Then, for the purpose of predispersion, the silica is incorporated into this mixture with the high-speed mixer (disk Ø 45 mm) and predispersed for 5 min at 2000 rpm. The mixture is dispersed in a laboratory pearl mill for 30 min at 2500 rpm and 60% pump capacity using glass beads (Ø approx. 1 mm). The millbase is tested with a grindometer, 25 μm, in accordance with DIN ISO 1524. It must be smaller than 10 μm.

The conversion of the millbase to lacquer takes place in accordance with the formulation, the components being mixed with a vane agitator at 2000 rpm. The hardener is incorporated in the same way.

After adjusting the lacquers to spray viscosity in accordance with DIN 53411, the lacquers are applied to black lacquered metal sheets, e.g. DT 36 (from Q-Panel), by spray application (coat thickness about 40-50 μm). After spraying, the metal sheets are dried for 24 h at room temperature and then for 2 h in a drying oven at 70° C.

Scratch Tests:

The metal sheets are abraded with a quartz/water slurry (100 g water+1 g Marlon A 350, 0.25%+5 g Sikron F500) using an abrasion and washing resistance tester (Erichsen, brush with hog's bristles). The gloss before and 10 min after the abrading is determined with a reflectometer (20° irradiation angle). TABLE 9 Summary of the properties of the liquid lacquers relevant in terms of lacquer technology, and of the applied and dried films. Reference Reference Comparative without Comparative Silica without silica 1 Silica 7 Silica 8 silica silica 2 Silica 9 11 silica Grindometer value [μm] <10 <10 <10 / <10 <10 <10 / Viscosity (millbase) [mPas]  6 rpm 409 210 220 / 5670 935 832 / 60 rpm 407 210 212 / 1260 409 407 / Viscosity (lacquer + hardener) [mPas]  6 rpm 120 80 80 60 446 195 175 55 60 rpm 113 82 82 61 194 146 144 64 Flow Poor OK OK OK Orange-peel OK OK OK fine cracks effect Scratch resistance 20° reflectometer 81 89.5 89.1 91.3 38 85.5 85.3 91.7 value before scratching Haze before scratching 101 9 12 2 423 18 19 2 40 strokes with Sikron 83.4 88.5 90.7 51.8 / 80.4 84.3 56.1 F 500 Residual gloss [%]

The silicas 7+8 and 9+10 according to the invention can be used in high concentrations without impairing the appearance of the lacquer surface owing to their substantially lower rheological efficiency compared with comparative silica 1 and 2. In addition, the silicas according to the invention display a substantial improvement in the scratch resistance of the lacquer surface.

Example 4

Direct comparison of the silicas according to the invention with a scratch-resistant lacquer according to DE 198 11 790 A1, in which AEROSIL R 972 is used to improve the scratch resistance. Silicas 2) Prior art according to the 1) invention Millbase Desmophen A 2009/1 190.2 Methoxypropyl acetate:Solvesso 36.8 100 1:1 Silica 23.0 Σ 250.0 Lacquer make-up Desmophen A YEP4-55A, 96.0 — contains AEROSIL R 972 Millbase — 48.9 Desmophen 2009/1 — 24.9 OL 17, 10% in MPA — — Modaflow 1% in MPA — — MPA:Solvesso 100 11.6 33.8 1:1 Butyl glycol acetate 10.5 10.5 Byketol OK 7.5 7.5 Byk 141 0.8 0.8 Addition of hardener Desmodur N 3390 23.6 23.6 Σ 150.0 150.0 Production and Application of the Lacquers

-   -   1) Comparative silica 1 is incorporated into the binder in         accordance with DE 198 11 790 A1 using a jet disperser.     -   2) The binder is mixed with the solvents. Then, for the purpose         of predispersion, the silica is incorporated into this mixture         with the high-speed mixer (disk Ø 45 mm) and predispersed for 5         min at 2000 rpm. The mixture is dispersed in a laboratory pearl         mill for 30 min at 2500 rpm and 60% pump capacity using glass         beads (Ø approx. 1 mm). The millbase is tested with a         grindometer, 25 μm, according to DIN ISO 1524. It must be         smaller than 10 μm.

The conversion to lacquer of the millbases corresponding to 1) or 2) takes place in accordance with the formulation, the components being mixed at 2000 rpm with a vane agitator. The hardener is incorporated in the same way.

After adjusting the lacquers to spray viscosity in accordance with DIN 53411, the lacquers are applied to black lacquered metal sheets, e.g. DT 36 (from Q-Panel), by spray application (coat thickness about 40-50 μm). After spraying, the metal sheets are dried for 24 h at room temperature and then for 2 h in a drying oven at 70° C.

Scratch Tests:

The metal sheets are abraded with a CaCO₃/water slurry (100 g water+1 g Marlon A 350, 0.25%+5 g Millicarb CaCO₃) using an abrasion and washing resistance tester (Erichsen, brush with hog's bristles). The gloss before and 10 min after the abrading is determined with a reflectometer (20° irradiation angle). TABLE 10 Summary of the properties of the liquid lacquers relevant in terms of lacquer technology, and of the applied and dried films. Prior art Silica 7 Reference Grindometer value <10 <10 / [μm] Viscosity (millbase) [mPas]  6 rpm 58 30 30 60 rpm 48 43 40 Surface Orange OK OK peel 20° reflectometer 88.0 86.5 98.5 value before scratching 100 strokes with 88.6 96.3 59.6 Millicarb Residual gloss [%]

It is shown that a substantially better improvement in the residual gloss is achieved after a scratch stressing of the lacquer surface by using the silica according to the invention than with the prior art. In addition, owing to its low rheological efficiency, the silica according to the invention does not cause an orange-peel effect. 

1. Silanised, structurally modified, pyrogenically produced silicas, characterised by groups fixed to the surface, wherein the groups are alkylsilyl (SiC_(n)H_(2n+1), with n=2-18).
 2. Silanised, structurally modified, pyrogenically produced silicas according to claim 1, characterised in that the pyrogenically produced silicas have been treated with the compound (CH₃O)₃SiC₁₆H₃₃ (hexadecyltrimethoxysilane).
 3. Silanised, structurally modified, pyrogenically produced silicas according to claim 1, characterised in that the pyrogenically produced silicas have been treated with the compound (CH₃O)₃SiC₈H₁₇ (octyltrimethoxysilane).
 4. Process for the production of the silanised, structurally modified, pyrogenically produced silicas according to claim 1, characterised in that the pyrogenically produced silicas are placed in a mixer, the silicas are sprayed, optionally first with water and then with the compound from the group (RO)₃SiC_(n)H_(2n+1) while mixing intensively, mixed for a further 15 to 30 minutes and then tempered at a temperature of 100 to 160° C. for a period of 1 to 3 hours, structurally modified and/or optionally post-ground.
 5. Process for the production of the silanised, structurally modified, pyrogenically produced silicas according to claim 4, characterised in that an additional tempering is allowed to follow the structural modification and/or post-grinding.
 6. Use of the silanised, structurally modified, pyrogenically produced silicas to improve the scratch resistance of lacquers.
 7. A silanised, structurally modified, pyrogenically produced silica, having alkylsilyl groups of the formula SiC_(n)H_(2n+1), with n=2-18.
 8. The silanised structurally modified, pyrogenically produced silica according to claim 7, having the following physical chemical properties: BET surface area 25-400 m²/g Average size of primary particles 5-50 nm pH value 3-10 Carbon content 0.1-25% DBP value in % at least 10% lower than the DBP value of a corresponding silianised, non-structurally modified silica
 9. The silanised, structurally modified, pyrogenically produced silica according to claim 7, wherein the pyrogenically produced silica has been treated with the compound (CH₃O)₃SiC₁₆H₃₃.
 10. The silanised, structurally modified, pyrogenically produced silica according to claim 7, wherein the pyrogenically produced silica has been treated with the compound (CH₃O)₃SiC₈H₁₇.
 11. A process for the production of the silanised, structurally modified, pyrogenically produced silica according to claim 7, comprising placing the pyrogenically produced silica in a mixer, spraying the silica, optionally first with water, and then spraying with a compound having the formula (RO)₃SiC_(n)H_(2n+1) wherein n is 2 to 18 and R is alkyl, while mixing intensively, mixing for a further 15 to 30 minutes and then tempering at a temperature of 100 to 160° C. for a period of 1 to 3 hours, structurally modifying and/or optionally post-grinding.
 12. The process for the production of the silanised, structurally modified, pyrogenically produced silica according to claim 11, further comprising additionally tempering following structural modification and/or post-grinding.
 13. The process according to claim 11, wherein structurally modifying takes place by ball milling.
 14. The process according to claim 11, wherein post grinding takes place by using an air-jet mill or pin mill.
 15. The process according to claim 12, wherein tempering takes place in a drying cupboard or in a fluidized bed.
 16. The process according to claim 15, wherein the tempering takes place under protective gas.
 17. A lacquer containing the silanised, structurally modified, pyrogenically produced silica of claim
 1. 18. A lacquer containing the silanised, structurally modified, pyrogenically produced silica of claim
 7. 19. A surface having applied thereto a coating produced from the lacquer of claim
 17. 20. The surface according to claim 19, which is metal. 